Heat exchangers include devices for transferring heat from a hot flowing stream to a cold flowing stream of material. Countercurrent heat exchange is the preferred arrangement for heat exchangers because then one can approach 100% heat recovery. With significant resistance to heat transfer, with deviation from plug flow, or with unequal flow of the two streams the heat recovery drops. With cocurrent heat transfer, even with negligible resistance to heat transfer, the average heat recovery for the two streams can never exceed 50%.
Since countercurrent heat exchange is much more efficient than cocurrent heat exchange it is aimed for and designed into heat exchangers whenever possible. With gases and liquids one can approach countercurrent exchange with no difficulty, the fluids are pumped in opposite directions, whether sideways or up and down. In gas/solid systems one can approach countercurrent heat exchange in various ways, such as with moving beds of downflowing solids combined with upflowing of gas, with raining solid contactors, staged fluidized beds, and other devices. U.S. Pat. Nos. 3,524,498; 3,705,620; 3,866,673 and 3,925,190 are examples of such systems.
In solid/solid systems the designer is faced with the difficulty that solids can only flow downward of themselves thus leading to cocurrent contacting and cocurrent heat exchange with its inherently low heat recovery.
Various methods have been proposed for overcoming this difficulty. When both streams consist of fine solids one may employ a third stream of solids consisting of large particles such as steel balls as heat carrier and go between. Thus in the first processing unit the falling steel balls pick up heat from the hot fines which are being transported upward pneumatically by a fast moving gas stream. The steel balls then give up their heat in the second similar unit to the upflowing cold fines. U.S. Pat. Nos. 4,110,193 and 4,157,245 are examples of such processes.
In principle these processes seem straightforward. In practice they are very complex systems requiring all sorts of mechanical seals, plus large gas flows to carry the solids upward. This absorbs much of the heat. In addition the upflowing solids will deviate greatly from plug flow thereby reducing the thermal efficiency drastically. Such systems entail considerable complexity when compared to this invention.